Some of the plugins provide overlapping functionality, like asciidoc and asciidoctor plugins. In that cases it suffices to install the dependencies for one of the overlapping plugins.
Usually a plugin has some pre-requisites to be used. If doconv detects in the system the dependencies needed for the plugin to work, the plugin will be enabled.
The asciidoc plugin converts from AsciiDoc to DocBook.
The asciidoctor plugin converts from AsciiDoc to DocBook.